1. Field of the Invention
The present invention relates to a mixer for directly sampling a radio frequency signal so as to frequency-convert the radio frequency signal, and for performing frequency selection.
2. Description of the Background Art
A charge sampling technique for sampling a radio frequency signal is disclosed in U.S. Pat. No. 5,414,311. In the technique disclosed in U.S. Pat. No. 5,414,311, a signal voltage is transformed into a signal current proportional to the signal voltage by using a transconductance amplifier, and the signal current is integrated in a capacitor for a predetermined time period so as to output the result as a sampling output, instead of an instantaneous voltage being sampled. After the integration is performed for the predetermined time period and the result is outputted, the capacitor is discharged. The integration and the discharging are periodically performed so as to obtain a discrete signal output. The technique disclosed in U.S. Pat. No. 5,414,311 allows a signal in a frequency band used for radio communication to be directly sampled.
In WO 01/24192, disclosed is a technique for forming a mixer having frequency selection characteristic of passing a signal in a desired frequency band and attenuating signals in frequency bands other than the desired frequency band by using the charge sampling technique. In the technique disclosed in WO 01/24192, the mixer having the frequency selection characteristic allows a radio signal receiver to have a simplified structure.
FIG. 13 shows a conventional circuit, disclosed in US Patent Application Publication No. 2003/0040294 and Japanese Laid-Open Patent Publication No. 2004-289793 (corresponding to US Patent Application Publication No. 2003/080888), in which the charge sampling technique is applied to a mixer having a frequency selection characteristic so as to increasingly enhance the frequency selection characteristic of the mixer. FIG. 14 shows a timing chart of an operation control signal used for the circuit shown in FIG. 13. The circuit is a combination of the charge sampling circuit with a circuit having IIR (infinite impulse response) filter characteristic. A circuit which has IIR filter characteristic and samples a signal is, for example, a switched capacitor circuit including an operational amplifier, a switch and a capacitor. On the other hand, in the circuit disclosed in US Patent Application Publication No. 2003/0040294 and Japanese Laid-Open Patent Publication No. 2004-289793 (corresponding to US Patent Application Publication No. 2003/080888), an IIR filter includes only a switch and a capacitor. Thus, it is unnecessary to provide an operational amplifier, which was a necessary component for a conventional switched capacitor circuit, thereby minimizing increase in consumption of power and circuit scale.
Further, in US Patent Application Publication No. 2005/0104654, disclosed is a technique for improving frequency selection characteristic by cascading IIR filters so as to increase an order (hereinafter, referred to as order increase) of an IIR filter, based on the techniques disclosed in US Patent Application Publication No. 2003/0040294 and Japanese Laid-Open Patent Publication No. 2004-289793 (corresponding to US Patent Application Publication No. 2003/080888). FIG. 15 shows a conventional circuit, disclosed in US Patent Application Publication No. 2005/0104654, in which circuits shown in FIG. 13 are cascaded so as to increase the order. A timing chart of an operation control signal used for the circuit shown in FIG. 15 is the same as the timing chart shown in FIG. 14. Also in this technique, the IIR filter includes only a switch and a capacitor, thereby improving frequency selection characteristic while minimizing increase in consumption of power and circuit scale.
Further, in David L. Fried, Analog Sampled-Data Filters”, IEEE Journal of Solid-State Circuits, August, 1972, a technique for forming a high-order IIR filter including only a switch and a capacitor is disclosed. In this technique, when one switch and one capacitor are added to the high-order IIR filter, the order of the IIR filter is incremented by one.
However, the conventional mixer as described above has a problem as descried below. That is, in the conventional arts (see FIG. 13) disclosed in US Patent Application Publication No. 2003/0040294 and Japanese Laid-Open Patent Publication No. 2004-289793 (corresponding to US Patent Application Publication No. 2003/080888), the order of the IIR filter is one, so that frequency selection characteristic is not sufficiently improved.
In the conventional art (see FIG. 15) disclosed in US Patent Application Publication No. 2005/0104654 in which the order of the IIR filter is increased by using the technologies disclosed in US Patent Application Publication No. 2003/0040294 and Japanese Laid-Open Patent Publication No. 2004-289793 (corresponding to US Patent Application Publication No. 2003/080888), frequency selection characteristic is improved while a loss (hereinafter, referred to as pass loss) of output signal level occurs in a signal passband. This is because an integration capacitor Cr2 and an integration capacitor Cr3 are periodically discharged in accordance with a discharge operation control signal RESET1 (see FIG. 14). FIG. 16 is a conceptual diagram illustrating the pass loss occurring in the signal passband in the conventional art disclosed in US Patent Application Publication No. 2005/0104654. As shown in FIG. 16, in the conventional art disclosed in US Patent Application Publication No. 2005/0104654, a pass loss occurs in a signal passband each time an order is increased, and the increase of the order leads to accumulation of the pass loss. US Patent Application Publication No. 2005/0104654 also discloses a technique for eliminating the pass loss. However, it is complicated to set filter characteristic so as to prevent an output of the IIR filter from interfering with an input thereof, resulting in the designing being difficult. Further, it is necessary to substantially increase the numbers of capacitors and switches so as to cascade the IIR filters for increasing the order thereof.
Further, in the conventional art disclosed in David L. Fried, “Analog Sampled-Data Filters”, IEEE Journal of Solid-State Circuits, August, 1972, a delay circuit or a control signal having a frequency higher than a sampling frequency is required so as to generate an operation control signal. There is a problem that, when a delay circuit is used, variations in delay time occur. In particular, since a plurality of operation control signals are required so as to increase an order of an IIR filter, it is substantially difficult to compensate the variations in delay times of the plurality of operation control signals. A logic circuit formed by using a technique of designing a synchronous circuit (a method for generating various signals based on a reference CLK) may be used so as to generate a stable operation control signal, instead of the delay circuit being used. However, the operation control signal used in this circuit is required to have a frequency higher than a sampling frequency. When a mixer using the sampling technique is used in a radio communication device, the sampling frequency ranges from several hundred MHz to several GHz. Thus, when the operation control signal having a frequency higher than a sampling frequency is used, the reference CLK generation circuit is complicated and consumption of current is increased, and further consumption of power in the logic circuit is increased.
Further, even a simple combination of the conventional arts described above cannot prevent increase in consumption of current caused by pass loss occurring due to the order increase and by the use of an operation control signal having a frequency higher than a sampling frequency.